The present invention relates to an optical scanning device, and more particularly to an optical scanning device for scanning a target member with a light beam that is emitted from a light source and deflected by the polygon mirror.
An electrophotography image forming apparatus uses an exposure device exposes the surface of a charged photosensitive member or photoreceptor to light beams in accordance with an image to be formed thereon. An optical scanning device is a typical known exposure device. The scanning device modulates a light beam emitted from a light source, e.g., a laser diode (LD), by information of an image to be formed; deflects the modulated light beam by deflecting means, e.g., a rotating polygon mirror; scans the surface of the photoreceptor with the light beam through the deflection of the light beam; and depicts an electrostatic latent image on the photoreceptor.
The Unexamined Japanese Patent Application Publication No. Hei 3-2773 discloses the following technique: in the optical scanning device, where a pulse width modulation (PWM) is applied to a laser light source to express a half tone, the increase of a contrast in a distribution of potential levels in an electrostatic latent image (contrast in a distribution of exposure amounts on the photoreceptor), leads to improvement of the toner characteristic in the PWM system. The publication also describes a further technique that the contract can be improved by reducing the beam diameter measured in the scan direction (fast scan direction) to 70% or less of the size of a recording pixel (pixel density).
Actually, it is almost impossible to technically satisfy the this condition. Recently, the PWM system is frequently adopted for forming an image at a high resolution of approximately 400 to 600 spi (=number of light spots per inch (density)). To satisfy the beam-diameter condition, the beam diameter must be 45 .mu.m or smaller for 400 spi, and it must be 30 .mu.m for 600 spi. It is considered that because of the focal depth in the optical system, the lower limit of the beam diameter is about 50 .mu.m for the laser beam of 780 nm in wave length, usually used, and it is about 43 .mu.m for the laser beam of 670 nm in the visible rays region. From this fact, it is seen that the maximum value of improved resolution that can be achieved by the technique is 400 spi at most.
The Unexamined Japanese Patent Application Publication No. Hei 9-169136 discloses an image forming technique as shown also in FIG. 12. As shown, a first light beam narrow in width but high in power is modulated by an image signal; a second light beam wide in beam width but low in power is modulated by an inverted image signal; the entire scan areal range is scanned (overwritten) with the first and second light beam, whereby an image is formed. Thus, an exposure amount distribution by the first light beam is superimposed on an exposure amount distribution by the second light beam, to form a composite exposure amount distribution (composite exposure image in FIG. 12). The contrast of the composite exposure image is improved, so that an electrostatic latent image of high potential contrast is produced.
The Unexamined Japanese Patent Application Publication No. Hei 9-169136 discloses four optical systems for realizing the image forming method as shown in FIGS. 13 and 14A to 14C. A first optical system shown in FIG. 13 includes a laser light source 100 for emitting a first laser beam and another laser light source 102 for emitting a second laser beam having a beam width different from that of the first laser beam, and a beam splitter 104 for superimposing those laser beams one on the other. The optical system depicts an electrostatic latent image on a photoreceptor with the superimposed light beam. A second optical system shown in FIG. 14A includes a dual spot laser 108 with two light emitting points as light sources from which two laser beams with different beam divergence angles .theta.1 and .theta.2 are emitted (laser 108=a laser array consisting of a pair of laser light sources closely arrayed), and one collimator lens 110 which receives two laser beams from the dual spot laser 108 and makes those laser beams collimated thereby. A third optical system shown in FIG. 14B includes a dual spot laser 112 with two light emitting points from which two laser beams with equal beam spread angles are emitted, and one collimator lens 110. As shown, in this optical system, the dual spot laser 112 is disposed while being slanted with respect to the collimator lens 110. A fourth optical system shown in FIG. 14C includes a dual spot laser 112 with two light emitting points from which two laser beams with equal beam spread angles are emitted, one collimator lens 110 and an optical element 114 having a refractive index different from that of air located in one of the optical paths between the dual spot laser 112 and the collimator lens 110.
The first optical system of FIG. 13 is disadvantageous in that it is very difficult to keep the landing positions of the paired laser beams on the photoreceptor invariable irrespective of aging and ambient variations. The optical systems of FIGS. 14A to 14C are advantageous in that the relative positions of the two light emitting points are fixed, and hence that it is relatively easy to keep the landing positions of those light beams invariable. However, the second optical system of FIG. 14A is disadvantageous in that the main body of the laser 108 needs to be finely worked to make the beam spread angles of the laser beams from two light emitting points different from each other. The third optical system of FIG. 14B has the following disadvantage: it is necessary to highly accurately slant the dual spot laser 112 with respect to the collimator lens when mounting the dual spot laser, and hence much and accurate work is essential. The fourth optical system of FIG. 14C has the following disadvantage: it is necessary to highly accurately dispose the optical element 114 between the dual spot laser 112 and the collimator lens 110, and hence much and accurate work is also required.